New Strategies in the Design of Paramagnetic CAs

Abstract

Nowadays, magnetic resonance imaging (MRI) is the first diagnostic imaging modality for numerous indications able to provide anatomical information with high spatial resolution through the use of magnetic fields and gradients. Indeed, thanks to the characteristic relaxation time of each tissue, it is possible to distinguish between healthy and pathological ones. However, the need to have brighter images to increase differences and catch important diagnostic details has led to the use of contrast agents (CAs). Among them, Gadolinium-based CAs (Gd-CAs) are routinely used in clinical MRI practice. During these last years, FDA highlighted many risks related to the use of Gd-CAs such as nephrotoxicity, heavy allergic effects, and, recently, about the deposition within the brain. These alerts opened a debate about the opportunity to formulate Gd-CAs in a different way but also to the use of alternative and safer compounds to be administered, such as manganese- (Mn-) based agents. In this review, the physical principle behind the role of relaxivity and the T1 boosting will be described in terms of characteristic correlation times and inner and outer spheres. Then, the recent advances in the entrapment of Gd-CAs within nanostructures will be analyzed in terms of relaxivity boosting obtained without the chemical modification of CAs as approved in the chemical practice. Finally, a critical evaluation of the use of manganese-based CAs will be illustrated as an alternative ion to Gd due to its excellent properties and endogenous elimination pathway.

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“…Examples of such nanosystems for the encapsulation of a specific contrast medium are widely available in the literature, with particular regard to MRI CAs, and showed to bring additional advantages like the improvement of the contrast-enhancing properties and the targeting capability obtained by means of surface decoration and functionalization. [79][80][81][82][83][84][85][86][87][88][89] More efforts are now devoted to developing nanocarriers for multimodal imaging purposes, especially for MRI/optical imaging, MRI/CT, and PET/MRI applications. 84,[90][91][92][93][94][95][96][97] However, in the case of PET/MRI, the development of these hybrid contrast media is particularly limited by the short half-life of PET tracers.…”

Contrasting Properties of Polymeric Nanocarriers for MRI-Guided Drug Delivery

“…Outer-sphere relaxivity is mainly described by translational diffusion due to the Brownian motion of free water molecules that interact with the electronic spins of the metal ions through dipolar intermolecular interactions. This process [87,88] depends on the distance of the closest approach of spins I and S (solvent protons and paramagnetic complex); N A is Avogadro's number, [M] is the molar concentration of the paramagnetic contrast agent, and D is the diffusion constant for relative diffusion [89].…”

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